1. Field of the Invention
The invention relates to a digital circuit arrangement for processing an analog video signal, which operates at a fixed system clock that is not coupled to the video signal, in which a digital video signal available at the system clock is processed and which comprises a correction memory and an interpolator with decimator which are used for converting the digital video signal to a synchronizing signal raster predetermined by the system clock.
2. Description of the Related Art
A circuit arrangement of this type is known from the publication "Fernseh- und Kino-Technik", vol. 40, No. 3/1986, pp. 105 to 111. In this publication the basic mode of operation of a television receiver operating at a free running clock is briefly described. Free running clock in this respect is understood to denote that the system clock at which the video signals converted into the digital mode are processed, is totally independent of the video signal and is thus independent of either its color burst frequency or its horizontal frequency. Above publication states that a buffer and an interpolator may be provided for eliminating the asynchronism between the input signal sampled at the system clock and a horizontal and vertical deflection raster derived from the free running system clock. A more detailed mode of operation of this buffer and interpolator cannot be understood from this publication.
European Patent Application EP-A 0,300,633 discloses an arrangement for time-base error correction which samples video signals at a fixed clock rate, processes them at the same rate and reconverts these signals into the analog mode. The purpose of that arrangement is to correct time-base errors in the analog input signal. The arrangement comprises an adaptive interpolation filter which is used for generating the values of the sampled video signals at the instants which would have been sampling instants if no time-base errors had occurred. For this purpose, the adaptive interpolation filter stores eight successive sample values of the digital video signal in a shift register. Each time four of the stored sample values are multiplied by evaluation factors and subsequently added together. In this manner a time-base error correction is made of which the time error is smaller than a sample period. In order to correct time errors, which are integer multiples of the sample period, the appropriate four sample values are selected from the sample values stored in the shift register. Thus, it is possible for this filter to perform a time-base error correction in integer multiples of the sample clock within a narrow time-dependent range of several sample values and also, by means of interpolation, to compensate for time-base errors which are smaller than a sample period.